Two of my best friends are dating and the guy tells me that he wants to kiss his girlfriend’s back. I reply:

lemme help you.
what you do is you get into close proximity with her. like about 5 cms, behind her.
the key point here, though, is to approach behind her whilst making as much noise as possible so that she knows that you are approaching.
because you see, there’s a thing called a “Reflex” where a startled Sacha might backhand the ugly out your face.
So, approach slowly and cautiously, whilst clapping your hands in a rhythmic, jolly manner to symbolize that you come in peace
when you are close enough to smell her hair, lean over her shoulder and say “hi,” in order to assure her that you are a civilized person.
when she looks up and says hi, lean backwards.
if she does not respond to your hi right away, (or jerks away to the sheer proximity of you two), step back and repeat the procedure.
Say she decides to reply your hi. usually what you want is a ‘hi’ or a smile of some sort from her, but we know that in your case, it’s highly unlikely, so if and when she gives any sign of recognition, put your hands on her hips.
caution, you do not want to grope.
if she has not pulled some ninja, feminist self defense move on you by now, keep your hands on her hips for another 10 minutes just to make sure.
when she has let down her guard and is conversing with her awesome Asian friend, then quietly extend your lips onto her shoulder.
caution: do not breathe heavily whilst doing this, because that will definitely call for a backhand from the awesome Asian friend.
let your lips extend onto her shoulder.
the kiss must not be lingering (to prevent drool or other germs escaping from that thing on your face thats dirtier than a dog’s anus.)
let it last for about 2 seconds
and what’s key here is that you RUN AWAY. vite.
before her grandma comes after you to tell you about safe sex.
it’s not the back, but the shoulder is closest thing; otherwise you’ll have to bend down quite a bit to get to her back, and chances are, 1) you are going to be in an awkward butt-extended position, leaving you in the mercy of rather rowdy Indian standbyers. Subclause 1). She will probably not feel it on her back anyway. subsubclause1) in order to prevent this kind of thing from happening, she is probably wearing her backpack to ward off any attempts on kissing her back.
voila, the art of seduction.”

Call me Sex kitten.